1. Field of the Invention
The present invention relates to a shape measuring apparatus, such as a coordinate measuring device, and to a shape measurement error correction method using the shape measuring apparatus.
2. Description of Related Art
Nowadays, in order to test accuracy of processing on a manufactured item having a three-dimensional shape, for example, a shape measurer such as a coordinate measuring device is used. A coordinate measuring device of this kind performs shape measurement by, for example, displacing a stylus tip of a scanning probe along the three-dimensional shape.
In a case where such a coordinate measuring device is used to perform measurement with a scanning probe, a measurement error may occur due to an effect accompanying movement of a slider to which the scanning probe is mounted. For example, when circle measurement is performed with the scanning probe, a motion error called “quadrant projection” occurs. “Quadrant projection” is a motion error forming projecting shapes when mechanically switching between quadrants of rectangular coordinates in the coordinate measuring device (when reversing a movement direction of each axis) in a case where the stylus tip of the scanning probe is performing circle measurement. Such quadrant projection occurs primarily due to backlash or the like caused by a mechanical structure of a measuring device.
FIG. 9 illustrates measurement results for a case where shape measurement is performed on a ring gauge (a ring-shaped measured object) using a generic coordinate measuring device. As shown in FIG. 9, peak-shaped measurement errors are observed in the measurement waveform at a border region P1 between a first quadrant and a fourth quadrant, as well as at a border region P2 between a second quadrant and a third quadrant. These are measurement errors caused by quadrant projection occurring due to backlash or the like when reversing the movement of the scanning probe in an X axis direction. Similarly, peak-shaped measurement errors are observed at a border region P3 between the first quadrant and the second quadrant, as well as at a border region P4 between the third quadrant and the fourth quadrant. These are measurement errors caused by quadrant projection occurring due to backlash or the like when reversing the movement of the scanning probe in a Y axis direction.
Meanwhile, an exemplary method of correcting scanning probe measurement error is described in Japanese Patent Laid-open Publication No. 2007-315897. According to this disclosure, a position of a forefront end of a slider is estimated using a correction filter based on frequency transfer characteristics between a scale and the forefront end of the slider. In addition, a measurement error occurring due to quadrant projection can be corrected by adding the estimated value to a scanning probe detected value and calculating a measured value.
In the coordinate measuring device described in Japanese Patent Laid-open Publication No. 2007-315897, a stage is provided on an anti-vibration table and an upper surface of the stage is made horizontal. A Y axis drive mechanism extending in the Y axis direction is installed at one X-axis-direction end of the stage. A beam support body is provided upright on top of the Y axis drive mechanism, and the beam support body can be driven in the Y direction by the Y axis drive mechanism. In addition, a beam is attached to a top end of the beam support body, the beam extending in a horizontal direction and in the X axis direction. A column is driven in the X axis direction along the beam. Furthermore, a slider is attached to the column so as to be drivable in a Z axis direction, and a scanning probe is mounted to a forefront end of the slider. In a coordinate measuring device having the above-noted configuration, a measured object (i.e. an object to be measured) is placed on the immobile stage and, in a state where the measured object is not expected to move, measurement of the measured object is performed by the scanning probe. In such a measuring apparatus, correction is performed by a calculator predicated on the freely displacing slider which has the scanning probe mounted on the forefront end thereof. However, when a correction method disclosed in Japanese Patent Laid-open Publication No. 2007-315897 is applied to a case where the measured object is measured by the scanning probe in a state where the measured object has been placed on a displacement table, there are instances where adequate correction cannot be performed.